The present invention relates in general to sewing machines and in particular to a new and useful sewing machine which is equipped for producing shaped seams.
Sewing machines are known which comprise feed means that are adjustable by a setting device and are equipped for producing shaped seams. The equipment includes a first sensor which is positioned ahead of the needle and senses a trailing edge of a workpiece extending at an angle relative to a leading edge thereof. A pulse generator coupled to the main shaft of the sewing machine and a program controlled switch circuit by which the sewing machine is stopped at a predetermined end point in response to the pulses issuing from the sensor and the pulse generator are provided.
Such a sewing machine for making shaped seams is disclosed in European OS No. 44 648. That machine comprises a positioning motor and equipment for producing corner seams. Corner seams are understood to be a species of the generic group of shaped seams. The equipment comprises a sensor which is provided ahead of the needle and serves the purpose of sensing a trailing workpiece edge extending at an angle to a leading edge, and for starting a counting of stitches formed. The equipment further comprises a microprocessor circuit controlling a positioning motor in order to stop the sewing machine as soon as the needle is stuck in the workpiece at a predetermined corner point.
While sewing a number of identical or similar workpieces, the sewing machine is operated manually during the sewing of the first workpiece, to establish a sewing program, with the number of stitches up to the response of the sensor, and the sewing speed being stored. While sewing the other workpieces, the operation of the sewing machine is controlled automatically by the microprocessor circuit, with the use of the data stored during the programming phase. By sensing the edges of each workpiece and thereby starting the stitch count in the end zone, disturbing factors such as unequal elongation of the workpieces and slippage between the feed mechanism of the sewing machine and the workpieces, are to be eliminated, to keep the accuracy of the seam length within the limits of .+-. one stitch length.
If a pulse generator coupled to the main shaft of the sewing machine is provided, which permits determination of the angular position of the main shaft or the instant during a stitch formation at which the sensor has responded to the passage of the respective workpiece edge, the microprocessor circuit determines the number of stitches made after the response of the sensor as a function of the difference between the angular position of the main shaft stored during the programming phase, and the angular position thereof measured while sewing the other workpieces. In this way, the seam length accuracy can allegedly be increased to .+-.0.5 stitch length.
This accuracy, however, cannot be attained if unequal feed conditions are caused with workpieces made of different materials, by unequal surface structure, of unequal thickness, or, with workpieces of identical material, by an unequal direction of the warp and filling threads of the tissue. That is, in such an event, a slip may occur between the feed mechanism of the sewing machine and the respective workpiece, which slip varies from piece to piece and produces the effect that the sewing program established during the manual sewing of the first workpiece no longer exactly fits the following workpieces. But even if, in the most favorable case, an accuracy of .+-.0.5 stitch length might be attained, a range of tolerance of a whole stitch length would be too large for sewing corners on pieces of garments where importance is attached to a flawless aspect of the seam and thus to an exactly formed end stitch of a seam, equidistantly spaced from either of the workpiece edges.